The telegenic young Cuban-American could potentially shore up three of the Romney campaign's weaknesses: He is an unambiguous conservative, elected with Tea Party backing, who would temper Romney's "Massachusetts moderate" image among the disgruntled GOP base. Rubio's groundbreaking candidacy could lure back some of the Hispanic voters driven away by Republican policies. And he happens to come from a huge swing state that Romney has to win in order to have a chance at the White House.

But Rubio would be a roll of the dice. How would he perform under the microscopic scrutiny that any candidate for national office must endure? Pitted against Vice President Joe Biden in a debate, would he seem callow and uninformed? Rubio could brighten Romney's prospects, but there's also a chance he could dim them considerably.

Wisconsin Rep. Paul Ryan: A safer choice, yet one that would restrict Romney's freedom to maneuver during the campaign.

Romney and Ryan have campaigned together this week, in what looked like Ryan's audition for the supporting-actor role, and there is an obvious rapport between them. Remarkably, it turns out that Ryan is even less charismatic on the stump than Romney — meaning there's no danger that Romney would be overshadowed.

But Ryan is the author of the House Republican budget, a document that has become the main target of President Obama's re-election campaign. Romney might figure that, having said nice things about the Ryan budget, he effectively owns it anyway. But there's a difference between owning the thing and chaining yourself to its creator.

New Jersey Gov. Chris Christie: A potential game-changer who could save the ticket or doom it — either way, spectacularly.

I think of Christie as the "fasten your seat belts" choice. He has credibility as a conservative Republican, yet manages to survive in a state where appealing to independents is crucial. And no other potential vice presidential candidate would fill the traditional "attack dog" role with more gusto.

But Jersey-style bombast wouldn't necessarily play well in many parts of the country — the places where people are, you know, polite — and Christie has a tendency to get carried away. One ill-timed eruption of invective could do fatal damage to the ticket's chances. And even if kept on a tight leash, Christie would be at pains to avoid drawing more attention to himself than to Romney.

Playing second fiddle to Mitt Romney won't be easy, but somebody has to be his running mate. Let's handicap the field:

The telegenic young Cuban-American could potentially shore up three of the Romney campaign's weaknesses: He is an unambiguous conservative, elected with Tea Party backing, who would temper Romney's "Massachusetts moderate" image among the disgruntled GOP base. Rubio's groundbreaking candidacy could lure back some of the Hispanic voters driven away by Republican policies. And he happens to come from a huge swing state that Romney has to win in order to have a chance at the White House.

But Rubio would be a roll of the dice. How would he perform under the microscopic scrutiny that any candidate for national office must endure? Pitted against Vice President Joe Biden in a debate, would he seem callow and uninformed? Rubio could brighten Romney's prospects, but there's also a chance he could dim them considerably.

Wisconsin Rep. Paul Ryan: A safer choice, yet one that would restrict Romney's freedom to maneuver during the campaign.

Romney and Ryan have campaigned together this week, in what looked like Ryan's audition for the supporting-actor role, and there is an obvious rapport between them. Remarkably, it turns out that Ryan is even less charismatic on the stump than Romney — meaning there's no danger that Romney would be overshadowed.

But Ryan is the author of the House Republican budget, a document that has become the main target of President Obama's re-election campaign. Romney might figure that, having said nice things about the Ryan budget, he effectively owns it anyway. But there's a difference between owning the thing and chaining yourself to its creator.

New Jersey Gov. Chris Christie: A potential game-changer who could save the ticket or doom it — either way, spectacularly.

I think of Christie as the "fasten your seat belts" choice. He has credibility as a conservative Republican, yet manages to survive in a state where appealing to independents is crucial. And no other potential vice presidential candidate would fill the traditional "attack dog" role with more gusto.

But Jersey-style bombast wouldn't necessarily play well in many parts of the country — the places where people are, you know, polite — and Christie has a tendency to get carried away. One ill-timed eruption of invective could do fatal damage to the ticket's chances. And even if kept on a tight leash, Christie would be at pains to avoid drawing more attention to himself than to Romney.

Louisiana Gov. Bobby Jindal: Plausible ... but why?

Romney has survived the primary campaign by declaring himself to be an ideologically pure conservative, but nothing in his history suggests this is actually the case. He's a dedicated believer in the free market, without question. Beyond that, however, he's basically a technocrat who lets data, not ideology, lead him to solutions — as with the health insurance mandate he instituted in Massachusetts.

So why would he choose another data-driven technocrat as his running mate? Jindal would bring neither charisma nor the electoral votes of a swing state to the ticket.

Virginia Gov. Bob McDonnell: Looked much better a few months ago.

McDonnell has it all — he's the moderate, good-looking governor of a key swing state. At least, that was his image before he placed himself on the front lines of what Democrats call the GOP's "war on women." In February, McDonnell supported state legislation that would have defined a fetus as a person and required women to submit to an intrusive vaginal ultrasound exam before having an abortion.

The "personhood" bill failed in the Virginia Senate, and McDonnell ended up supporting — and signing into law — a measure that mandates an ultrasound before abortion but allows women to opt for a non-intrusive procedure. But with female voters deserting the party in droves, would Romney want to ensure that the words "war on women" are on every Democrat's lips until Election Day?

Ohio Sen. Rob Portman, Indiana Gov. Mitch Daniels, South Dakota Sen. John Thune, former Minnesota Gov. Tim Pawlenty: Safe, safe, safe, safe. Not much excitement here, which may be the way Romney wants it. Maybe a slight edge to Portman because he's from Ohio, another of those must-win swing states.

South Carolina Gov. Nikki Haley: Given her talent for controversy, I'm begging on behalf of columnists everywhere, Mr. Romney. Please. Make our day.

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